1. Field of the Invention
The present invention relates to a process for producing hematite for ironmaking.
2. Description of the Related Art
More particularly, the present invention relates to a technique for separating a tailings slurry obtained after a nickel oxide ore is treated though a final neutralization step according to a high pressure acid leach (HPAL) process in a hydrometallurgical refining plant, thereby recovering hematite for ironmaking.
Nickel has been widely used as a raw material of stainless steel, but in accordance with the tendency of depletion of a sulfide ore that becomes a raw material of nickel, a technology of refining a low-grade oxide ore has been developed and has been into practical use.
Specifically, a producing process called a “high pressure acid leach (HPAL)” process has been put into practical use, in which process a nickel oxide ore such as limonite and saprolite is put into a pressurizing apparatus such as an autoclave in combination with a sulfuric acid solution, and nickel is leached under a high temperature of approximately 240° C. to 300° C. and a high pressure. FIG. 3 illustrates a schematic flowchart of the producing process.
A neutralizing agent is added to nickel that is leached in the sulfuric acid solution in the HPAL process so as to neutralize a surplus acid, and then nickel is subjected to solid-liquid separating treatment so as to be separated as a leach residue.
Then, the nickel is recovered as an intermediate raw material in a type of a hydroxide, a sulfide, and the like through a process of separating impurities, and the intermediate raw material is further refined to obtain nickel metal, nickel salt compound, and the like.
In a process of neutralizing the surplus acid, pH of a leach substance is adjusted to be appropriate for the solid-liquid separation, and then in a solid and liquid separation process that is a subsequent process, concentration of a solid content and solid and liquid separation are performed in a facility called counter current decantation (CCD). Typically, in CCD, a plurality of continuous stages of thickeners are used.
A liquid component (hereinafter, may be referred to as an overflow), which is obtained from CCD, is returned to a neutralization process for adjustment of pH that is appropriate for a sulfurization process. In the neutralization process, pH is adjusted so as to remove a fine solid content, which occurs, through precipitation. Then, for example, the liquid component is subjected to a sulfurization treatment, whereby an intermediate raw material such as a nickel sulfide is obtained.
Employing the HPAL process makes it possible to leach nickel almost completely, for example, in the case of nickel oxide ore, even in a low grade ore in which a target valuable metal to be recovered is contained in an amount of 1% by weight to 2% by weight (hereinafter, the weight will be expressed by “%”). Accordingly, a target metal is concentrated to the same extent as in a conventional raw material, and the target metal can be obtained through substantially the same refining method and refining process as in the conventional raw material. In addition, the HPAL process is applicable to not only the nickel oxide ore but also different raw materials, such as a nickel sulfide ore, a copper sulfide ore, and a copper oxide ore.
In addition, a main component of the leach residue that is obtained in the HPAL process is an iron oxide, and iron in the solid-content leach residue is approximately 40 to 50%. In addition, an amount of the leach residue that is produced is approximately 50 times to 100 times an amount of the intermediate raw material that is produced. The reason for this is that each of the nickel oxide ore or the sulfide ore that is a raw material contains iron in an amount much more than that of nickel.
The leach residue is generated at a high temperature, and is in a type of a chemically and environmentally stable oxide, but has no particular utility value in a current state, and is accumulated and stored in residue accumulation grounds.
Therefore, a broad residue disposal yard is necessary for scrap and storage of an enormous amount of the leach reside which is generated in accordance with the HPAL process operation.
By the way, many of the iron oxide is contained in an iron ore and the iron ore is widely used as a raw material for refined steel.
In iron and steel refining, the following process is used. An iron ore containing an iron oxide is charged into a blast furnace in combination with a reducing agent such as cokes, and the iron ore is reduced and melted through heating, thereby forming crude steel. The crude steel is refined in a converter furnace so as to manufacture target steel.
Typically, the iron ore as the raw material is a limited resource, and availability of an iron ore with good quality that is necessary for quality maintenance of steel becomes gradually difficult. Accordingly, an examination has been made with respect to use of the leach residue as the iron ore.
However, the leach residue in the HPAL process cannot be directly used for a raw material for ironmaking from the following reasons.
The leach residue in the HPAL process contains vein stone or impurities, particularly sulfur, in addition to the iron oxide, and thus the leach residue is not appropriate for a raw material that is used in an iron producing process in the related art. Specifically, the sulfur content is high.
Particularly, the sulfur content in the iron oxide which can be used for a raw material for ironmaking is different depending on facility capacity, an amount of production, and the like in individual ironworks. However, typically, it is necessary to suppress the sulfur content to less than 1%.
On the other hand, the solid-content leach residue contains approximately 5% to 8% of sulfur. However, the majority of sulfur in the leach residue is derived from calcium sulfate (so-called gypsum) that is mixed in during the HPAL process.
When neutralizing free sulfuric acid (the free sulfuric acid is sulfuric acid that remains without reaction in the sulfuric acid that is excessively added for performing sufficient leaching in the HPAL process), which remains in leach slurry that is obtained during high-pressure acid leaching, an inexpensive calcium-based neutralizing agent which is usually used, for example, limestone or slaked lime is added. Accordingly, when calcium contained in the neutralizing agent and the free sulfuric acid react with each other, the gypsum is generated, and is mixed into the leach residue.
A part (approximately 1%) of sulfur that is contained in the solid-content leach residue is trapped inside particles of hematite that is generated.
The solid content in the residue after nickel leach obtained at this point is mainly constituted by hematite having a particle size of approximately 1 μm, and the solid content contains the iron content of approximately 30 to 40% and the sulfur content of approximately 5 to 8%. The moisture content of the leach residue obtained at this point is 60%.
In order to use the leach residue as the hematite for ironmaking, it is necessary to achieve the iron grade of 50% or more and the sulfur grade of 1% or less in the solid-content leach residue.
As a technique to achieve this, for example, JP 2010-095788 A discloses a technique for removing impurities contained in a mixture of hematite by separating a leach residue by sieve classification, wet-cyclone separation, and magnetic separation, and the technique has been recognized to have some effect on removal of impurities contained in hematite.
However, the process is not enough to use the leach residue after the physical separation alone as hematite for ironmaking as described above, and in particular, the iron content only increases to approximately 40 to 45% at most. Accordingly, in order to use such a leach residue as hematite for ironmaking, it was necessary to mix with a raw material for ironmaking which contains a higher grade of iron. The moisture content of the leach residue obtained by physical separation is approximately 40%.
Furthermore, even though such reduction of sulfur grade is achieved, performing dehydration using a typical filter press or the like causes a problem of liquefaction during a transfer process, since moisture of approximately 22% is contained in the resultant residue after nickel leach (hematite cake).
In order to remove the moisture contained in the residue, there are various methods including sun-drying, heating, roasting, or centrifugal separation. However, when the moisture is excessively reduced, there are problems that dust is generated and thus transfer handling becomes difficult.
Therefore, a method of reducing such moisture to an optimum level and having operational efficiency has been required.